There have been, heretofore, two types of heat exchangers for drying moist tacky materials to supplement the disadvantages of a direct heating type. One heat exchanger is of a screw conveyor type which supplies heat medium to a hollow portion of a rotor and thermally exchanges materials while feeding by a spiral follow continuous blade provided on the outer periphery of the rotor a material toward a direction of a rotational shaft. The other heat exchanger is of a thermal disc type which aligns a number of hollow discs of triangular cross section on a rotor and thermally exchanges materials via heat medium supplied into the hollow discs.
The former screw conveyor type has such disadvantages as small thermal exchanging area per unit per unit volume of its housing and small treating capacity.
The latter thermal disc type has various disadvantages that hollow discs are excessively large to reduce the effective area for containing materials, the hollow discs of triangular cross section cannot effectively agitate nor feed the materials as the materials are moist and tacky like organic materials, and tend to adhere to the surfaces of the hollow discs, to stay as large lump between the discs. The materials are thus locally heated, resulting in a difficulty in efficient thermal exchange and in a deterioration of the quality of the materials. Further, air (remaining air, noncompressive gas in the discs) and drain (condensate) are not recovered by centrifugal force in the triangular space at the outer peripheral side in the rotating hollow discs but remain. Thus, the heat medium is not sufficiently supplied and thermal exchange is disturbed by the residue. Further, thermal conductivity decreases due to the materials adhered to the surface of the hollow discs and thus reduces the coefficient of thermal transmission. Consequently, heat medium of high pressure is supplied into the hollow discs. Thus, a number of posts must be welded fixedly in the hollow discs so as to meet the safety standard as a pressure vessel in strength due to the regulations of a boiler. Therefore, some portions of the outer peripheral edges of the hollow discs which are to contribute mostly to the heat exchange cannot transmit the heat and decreases the thermal efficiency. Further, the discs corrode in the portions which cannot transmit the heat, thereby eventually causing the heat medium to be externally leaked.
In order to eliminate the disadvantages of such a conventional heat exchanger, Japanese Patent Publication No. 41501/1977, (corresponding to U.S. Pat. No. 3,923,097, describes a heat exchanger in which annular ribs cut from plane sheets are attached to the outer periphery of a rotor, a spiral duct closed at the outer end is formed at one side of the rib, the interior of the duct is longitudinally divided to form reciprocating paths of the heat medium or heat medium and condensed water are recovered through passages formed radially of the ribs from the closed outer end of the duct to circulate in the duct.
However, the heat exchanger of this type still has the following drawbacks.
First, since the fixture of the rib formed from the plane sheet to the rotor is not welded fixedly to the outer periphery of the rotor through a wide gap at the lower ends of the disc like the conventional hollow disc, the strength of the rib is small.
Second, the heat medium and the condensed water from the closed end of the outer periphery of the spiral duct are intended to be unreasonably recovered from the outer peripheral end having high peripheral velocity to the rotor of the central direction against the centrifugal force due to the rotation of the rib, and air and drain cannot be sufficiently recovered. Thus, the heat medium must be supplied into the duct under as high pressure as possible.
Third, the formation of the spiral duct or of the ribs on the duct requires extremely complicated steps so as to maintain high accuracy and strength.
Fourth, the portions which do not transmit heat are produced at the closed end of the duct and at the outer periphery of the rib, and the portion which does not transmit heat is widely presented at the peripheral edge of the rib which mostly contribute to the heat exchange, resulting in a low thermal efficiency. Since corrosion occurs from the portion which does not transmit heat of the rib, the closed end of the duct is eventually corroded, thereby resulting in the leakage of the heat medium from the corroded portion.